The present invention relates to a pretensioner for a seat belt system and, in particular, to a pretensioner in which a belt take-up reel is driven in rotation to wind a segment of the seat belt onto it by a rotary actuator operated by gas pressure supplied by a gas generator.
Seat belt pretensioners are often provided in vehicle seat belt systems to tighten the seat belt at the instant of a collision or other emergency and thereby more firmly restrain the seat occupant against being thrown forward. One known type of seat belt pretensioner employs a rotary actuator that is supplied with gas under pressure from a gas generator and is coupled to a belt take-up reel. The gas from the generator is supplied to an annular cylinder chamber of the actuator and drives the rotor in rotation, thereby rotating the take-up shaft. A pretensioner of the type operated by a rotary actuator is disclosed in Japanese Patent Publication No. 60-15657.
In previously known pretensioners that are based on rotary actuators as the driving mechanism, leakage of the pressurized gas from the pressure section of the cylinder at various places results in a loss of the available energy of the gas. Minimizing the leakage of gas has heretofore been accomplished by maintaining close dimensional tolerances in the manufacture of the components of the actuator. Even with the maintenance of close dimensional tolerances, which increases manufacturing costs, leakage of the gas is still a problem.
In previously known rotary actuator-operated pretensioners, moreover, the rotor is mounted on the cylinder by reception of end portions of the ring body in holes in the cylinder. In that arrangement, the width of the rotor vane is reduced compared to the width in the direction of the axis of rotation of the rotor by an amount equal to the total of the widths of the portions of the rotor body that extend out into the receiving holes of the cylinder. In order to maintain a given force on the vane, it is necessary, therefore, for the actuator to have a width somewhat greater than the sum of the width of the vane plus the widths of the portions of the rotor ring body that are received in the holes in to cylinder side walls.